In the operation of a vessel or pipe carrying a flow of natural water--such as a heat exchanger--a slimy biofilm will develop on the submerged metal surfaces. This biofilm gradually thickens with time; soon it significantly interferes with heat exchange and the volume of flow which can be accommodated. Also, the biofilm deposit greatly increases the frictional resistance to flow within the fouled pipes. Pumping efficiency through the pipes is therefore significantly reduced.
Over the last few years, the development and nature of the biofilm have been studied and may be described as follows: Individual bacteria cells present in the water generate an outwardly extending mass of polysaccharide fibres. This felt-like mass is termed a "glycocalyx". The fibres function to anchor the cell to the metal surface and also to the glycocalyces of other cells. In time, a complex population of bacteria encapsulated within a matrix of fibres is generated and adheres tenaciously to the submerged surface. A more detailed explanation of the foregoing is given in the article "How Bacteria Stick" by J. W. Costerton, G. G. Geesey and K.-J. Cheng in the January, 1978, issue of Scientific American, Vol. 238, No. 1, pages 86-95.
Most attempts to alleviate this problem have been directed toward removing the biofilm, once it is established on the metal surface.
Recent techniques to remove established biofilm have involved the use of chlorine, biocides or mechanical abrasion. Chlorine works by killing the bacteria and oxidizing the polysaccharides to break the film away from the metal surface. However, while chlorine is effective, its presence in the effluent is environmentally undesirable. The biocides, such as various aldehydes, operate by killing bacteria and thereby reduce the rate of growth of the biofilm. However, it appears that the biocides have difficulty in penetrating the glycocalyx--thus inordinate quantities of the chemicals have to be used to achieve any effect on the biofilm. Mechanical abrasion has been tried by passing `pigs` or balls, coated with abrasive material, past the surface, to dislodge the biofilm. However, this is a labour intensive and thus expensive technique.
It follows, therefore, that there is a need for a simple and effective way to remove biofilm from a submerged fouled surface in an industrial aquatic system. In the case of a heat exchanger, this will improve or restore heat exchange efficiency and permit free water flow.
Prior art of interest is U.S. Pat. No. 661,873. It describes cooling a boiler inner surface to below the freezing point of water, to thereby contract the metal and expand the scale. This causes the scale to break away from the metal wall. This disclosure is directed to a material differing significantly from biofilm. More particularly, scale is a crystalline mass of insoluble inorganic salts while biofilm is composed of bacterial cells and of their organic products.